A Quine in Clojure

According to Wikipedia, a quine is a non-empty computer program which takes no input and produces a copy of its own source code as its only output.
In other words, a quine is a code snippet whose output is exactly the same as its source code.
In the Clojure world, we would say that a quine is a code expression that evaluates to itself.

We say code expression in opposition with data expressions that trivially update to themselves:

42
; 42

[1 2 42]
;[1 2 42]

TL;DR

If you have no patience to read this article, here is the simplest Clojure Quine:

((fn [x] (list x (list 'quote x))) '(fn [x] (list x (list 'quote x))))

Indeed, the output is exactly the same as the source code.

I hope that now you are motivated to learn, step by step, how we this quine was discovered.

Quine: first attempt

Like we wrote before, a quine is a piece of code that evaluates to itself. As a first attempt, we might be tempted to try to write a function that returns its source code as a form. After all, Clojure is homoiconic: the structure of the code is the same as the structure of the data.

Let’s try to write an anonymous function with no arguments that returns its code as a quoted form. This function does nothing so its code should be (fn []). So let’s write and call an anonymous function that returns '(fn []):

((fn [] '(fn [])))

The problem is that now our anonymous function doesn’t do nothing. In fact, it returns '(fn []). So the body of the function is now '(fn []).

No problem, let’s improve our anonymous function so that it returns '(fn [] '(fn [])):

((fn [] '(fn [] '(fn []))))

But now, the problem is that our anonynous function returns '(fn [] '(fn []).

This strategy is not going to work: we will always be missing a (fn []) wrapping level.

Quine: self referentiality

You might already have guessed that a real quine is going to involve self-referentiality.

Let’s find a function f and an argument x such that such that (f x) is exactly (f x).

A good guess for x is 'f.

So let’s write our function f such that (f 'f) is (f 'f):

(defn f [x] '(f 'f))
(f 'f)

It works! (f 'f) indeed evaluates to itself.

The only problem is that our code contains the definition of f but our output doesn’t.

In order to overcome this issue, we need to define f as an anonymous function.

As a first step, let’s rewite f in such a way that it will return (f 'f) not for all the arguments but only when we pass 'f to f:

(defn f [x] (list x (list 'quote x)))
(f 'f)

Finally if we replace f by its definition, we get our quine:

((fn [x] (list x (list 'quote x))) '(fn [x] (list x (list 'quote x))))

Do you feel a headache?

That’s normal: self-referentiality is very often a dizzy activity.

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By the time you finish your final capstone project, you’ll be designing Clojure functions, apps, and libraries like a pro!